package 测试;

import java.math.BigDecimal;
import java.util.Arrays;
import java.util.Scanner;

public class 路灯 {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		while (sc.hasNext()) {
			int n = Integer.parseInt(sc.next());
			long l = sc.nextLong();
			long[] a = new long[n];
			for (int i = 0; i < n; i++) {
				a[i] = Integer.parseInt(sc.next());
			}
			System.out.println(func(n, l, a));

		}
		sc.close();
	}

	public static String func(int n, long l, long[] a) {
		Arrays.sort(a);
		double dist = 0;

		for (int i = 0; i < a.length - 1; i++) {
			dist = Math.max(a[i + 1] - a[i], dist);
		}

		dist = Math.max((l - a[a.length - 1]) * 2, dist);
		dist = Math.max(a[0] * 2, dist);

		BigDecimal bd = new BigDecimal(dist / 2).setScale(2);
		return bd.toString();
	}

}
